The present invention relates to a method of effecting epitaxial growth which suppresses deformation of a surface of a substrate which has corrugations formed thereon.
In parallel with improvements recently attained in transmission systems in optical fiber communications, there is an increasing demand for semiconductor lasers which lase in a single longitudinal mode. Extensively studied today, in an attempt to meet such a demand, are distributed feedback laser diodes (DFB LD) and distributed Bragg reflector laser diodes (DBR LD).
For fabricating semiconductor lasers with the above-mentioned feature, it is necessary to effect epitaxial growth on a grating substrate which has corrugations formed on its surface at a period of several thousands of Angstroms. What is important for epitaxial growth is preventing the corrugations on the grating substrate from being deformed. It is generally observed, however, that the corrugations are thermally deformed while held at a high temperature before epitaxial growth or deformed by the meltback of the surface in liquid phase epitaxial (LPE) growth.
Especially, in the case of an InGaAsP system employing InP as a substrate which is capable of forming a semiconductor laser in a wavelength range of 1.1-1.6 microns, an important range for optical fiber communications, dissociation of P from the surface of the InP semiconductor layer or from the surface of the InGaAsP layer is considerable and, therefore, how to protect the grating before epitaxial growth is the critical problem.
One approach heretofore proposed to solve the above problem involves closely covering a grating substrate with an InP crystal (K. Sakai et al., "1.5 .mu.m Range InGaAsP/InP Distributed Feedback Lasers", IEEE JOURNAL OF QUANTUM ELECTRONICS, August 1982, VOL. QE-18, No. 8, pp. 1272-1277) or with a GaAs crystal (J. Kinoshita et al., "Preserving InP Surface Corrugations for 1.3 .mu.m GaInAsP/InP DFB Lasers from Thermal Deformation during LPE Process", ELECTRONICS LETTERS, Mar. 17, 1983, Vol. 19, No. 6, pp. 215-216). Another known approach involves protecting a grating substrate in an atmosphere of P which evaporates from a Sn--P solution, utilizing the fact that a large amount of InP dissolves in a Sn solution (G. A. Antypas, "Prevention of InP Surface Decomposition in Liquid Phase Epitaxial Growth", Appl. Phys. Lett. 37(1), July 1, 1980, pp. 64-65). Still another known approach is introducing PH.sub.3 in a reactor tube to elevate the P pressure (Nagai et al., "Deformation and Preservation of Diffraction Grating for Long Wavelength Band DFB Lasers", Lecture No. 6a-S-1, Manuscripts of 30th Joint Meeting of Applied Physics, Spring 1983, pp. 635 (in Japanese) or Nagai et al, "Prevention of Surface Corrugation Thermal Deformation for InGaAsP/InP DFB Lasers", Japanese Journal of Applied Physics, Vol. 22, No. 5, May, 1983, pp. L291-L293). However, each of the described approaches has both merits and demerits and more convenient and effective implementations have been demanded.